io_uring: avoid taking ctx refs for task-cancel

Don't bother to take a ctx->refs for io_req_task_cancel() because it
take uring_lock before putting a request, and the context is promised to
stay alive until unlock happens.
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1985,10 +1985,10 @@ static void io_req_task_cancel(struct callback_head *cb)
 	struct io_kiocb *req = container_of(cb, struct io_kiocb, task_work);
 	struct io_ring_ctx *ctx = req->ctx;
 
+	/* ctx is guaranteed to stay alive while we hold uring_lock */
 	mutex_lock(&ctx->uring_lock);
 	__io_req_task_cancel(req, req->result);
 	mutex_unlock(&ctx->uring_lock);
-	percpu_ref_put(&ctx->refs);
 }
 
 static void __io_req_task_submit(struct io_kiocb *req)
@@ -2019,14 +2019,12 @@ static void io_req_task_queue(struct io_kiocb *req)
 	ret = io_req_task_work_add(req);
 	if (unlikely(ret)) {
 		req->result = -ECANCELED;
-		percpu_ref_get(&req->ctx->refs);
 		io_req_task_work_add_fallback(req, io_req_task_cancel);
 	}
 }
 
 static void io_req_task_queue_fail(struct io_kiocb *req, int ret)
 {
-	percpu_ref_get(&req->ctx->refs);
 	req->result = ret;
 	req->task_work.func = io_req_task_cancel;